U.S. patent number 5,147,823 [Application Number 07/707,236] was granted by the patent office on 1992-09-15 for method for forming an ultrafine metal pattern using an electron beam.
This patent grant is currently assigned to Sony Corporation. Invention is credited to Kenji Funato, Akira Ishibashi, Yoshifumi Mori.
United States Patent |
5,147,823 |
Ishibashi , et al. |
September 15, 1992 |
Method for forming an ultrafine metal pattern using an electron
beam
Abstract
In a method for forming a pattern, by selectively irradiating a
charged particle beam onto a substrate in an atmosphere containing
a raw material gas, a resist pattern comprising a material which is
produced on the substrate from the raw material gas is formed,
wherein a pressure of the raw material gas is set to 10.sup.-7 to
10.sup.-5 Torr, an accelerating voltage of the charged particle
beam is set to 0.5 to 6 kV, and a beam current of the charged
particle beam is set to 10.sup.-13 to 10.sup.-7 A. Thus, a resist
pattern of an ultrafine width can be stably formed in a relatively
short time. Further, in a method for forming a pattern, by
irradiating a charged particle beam onto a substrate in an
atmosphere containing a gaseous negative type resist, a
cross-linking reaction of the negative type resist molecules
adsorbed on the surface of the substrate is caused, and a pattern
comprising the negative type resist molecules which caused the
cross-linking reaction is formed, so that a pattern which has an
ultrafine width and can be easily removed by the wet process can be
formed.
Inventors: |
Ishibashi; Akira (Kanagawa,
JP), Mori; Yoshifumi (Chiba, JP), Funato;
Kenji (Kanagawa, JP) |
Assignee: |
Sony Corporation (Tokyo,
JP)
|
Family
ID: |
26537116 |
Appl.
No.: |
07/707,236 |
Filed: |
May 22, 1991 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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412479 |
Sep 26, 1989 |
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Foreign Application Priority Data
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Sep 20, 1988 [JP] |
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63-245224 |
Oct 21, 1988 [JP] |
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63-265622 |
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Current U.S.
Class: |
438/694;
148/DIG.46; 250/492.1; 250/492.3; 430/323; 430/324; 438/758;
438/949 |
Current CPC
Class: |
G03F
7/004 (20130101); G03F 7/2041 (20130101); Y10S
438/949 (20130101); Y10S 148/046 (20130101) |
Current International
Class: |
G03F
7/004 (20060101); G03F 7/20 (20060101); H01L
021/00 (); H01L 021/02 (); H01L 021/306 () |
Field of
Search: |
;437/225,228,229,233,928,935 ;148/DIG.46 ;430/5,323,324
;250/492.1,492.2,492.3 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0077445 |
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Apr 1983 |
|
EP |
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0171068 |
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Feb 1986 |
|
EP |
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0318037 |
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Apr 1989 |
|
EP |
|
3015034 |
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Oct 1981 |
|
DE |
|
0046372 |
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Mar 1985 |
|
JP |
|
0010241 |
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Jan 1986 |
|
JP |
|
0042417 |
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Feb 1987 |
|
JP |
|
0152120 |
|
Jun 1988 |
|
JP |
|
Other References
Wolf, S., Silicon Processing for the VLSI Era, vol. 1, pp. 420-421,
1986, Lattice Press. .
Gangal, S., Plasma-Poly. Electron Beam Resist Prepar. from Methyl
Metacrylate Using Various Carrier Gases, pp. 341-350, Thin Solid
Films, 149 (1987). .
Morrissey, J., Electron-Beam Contaminantion as a Mask, IBM Tech.
Discl. Bull., vol. 20, No. 6, Nov. 1977, p. 2212. .
Japanese Patent Abstract, vol. 10, No. 150, (E-408) [2207], May 31,
1986, (11) 61-10241 (A). .
IBM Technical Disclosure Bulletin, vol. 20, No. 6 Nov.
1977..
|
Primary Examiner: Hearn; Brian E.
Assistant Examiner: Everhart; B.
Attorney, Agent or Firm: Hill, Van Santen, Steadman &
Simpson
Parent Case Text
This is a continuation of application Ser. No. 412,479, filed Sep.
26, 1989, abandoned.
Claims
What is claimed is:
1. A method for forming an ultrafine metal pattern, comprising the
steps of:
providing a substrate on which a metal film is formed, said
substrate and metal film being provided in a vacuum specimen
chamber;
introducing a gaseous negative type resist for an electron beam
into the specimen chamber, a vapor pressure of the negative type
resist being in a range from 10.sup.-7 to 10.sup.-5 Torr;
providing a vacuum in said specimen chamber, and forming a negative
type resist molecular layer comprising negative type resist
molecules absorbed on a surface of the metal film;
irradiation an electron beam whose beam diameter is finely
converged to approximately 100 .ANG. or less onto the metal film in
the gaseous negative type resist atmosphere while scanning a
desired pattern, an accelerating voltage of the electron beam being
in a range from 0.5 to approximately 6 kV, and having a beam
current in a range from 10.sup.-13 to 10.sup.-7 A;
molecules in said negative type resist molecular layer where said
electron beam is irradiated causing a cross linking reaction;
repetitively executing scanning of the electron beam for said
desired pattern until a thickness of the desired pattern which was
initially irradiated and comprising the negative type resist
molecules which caused the cross linking reaction reaches a desired
thickness;
selectively removing portions of the negative type resist molecular
layer to which the electron beam was not irradiated by a wet
process using a solvent for the negative type resist, a resist
pattern remaining which comprises the negative type resist
molecules which caused the cross linking reaction, and a width of
said resist pattern being substantially equal to said beam diameter
of the electron beam;
aniostropically etching the metal film while suing the resist
pattern as a mask so as to form a metal ultrafine line of 100 .ANG.
or less; and
removing the resist pattern by a wet process using a solvent.
2. A method according to claim 1 wherein the metal layer is
tungsten.
3. A method according to claim 1 wherein the vapor pressure of the
negative type resist is approximately 10.sup.-6 Torr.
4. A method according to claim 1 wherein the accelerating voltage
of the electron beam is approximately 6 kV.
5. A method according to claim 1 wherein the beam current of the
electron beam is selected so as to obtain an irradiation charge
density which is substantially equivalent to a sensitivity of the
negative type resist which is used.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for forming a pattern
which is suitable for use in formation of a fine pattern in, for
instance, the manufacturing of semiconductor devices.
2. Description of the Prior Art
Hitherto, as techniques to form a resist pattern, photolithography
using light, and electron beam lithography using electron beams,
are used. In the photolithography and electron beam lithography,
five steps of coating a resist, baking, exposing, developing, and
post baking are necessary; and the wet process is used.
In the conventional photolithography as described above, since the
a resolution of which is not longer than the wavelength of the
light which is used for exposure cannot be obtained, the minimum
pattern width which can be formed is limited to about 0.25 .mu.m.
On the other hand, the use of X-ray lithography using X-rays whose
wavelength is short and is about a few .ANG. in place of light has
also been known. However, in this case, a large scale apparatus
such as a synchrotron orbital radiation (SOR) apparatus is
necessary as a light source. In addition, there is a problem that
the construction of the optical system such as lens, mask, and the
like is generally difficult.
In the conventional electron beam lithography as described above,
when an electron beam is irradiated onto an electron beam resist,
the trace of the electrons is made random by multiple scattering in
the electron beam resist, so that the inherent resolution of the
electron beam is lost. Therefore, the width of the pattern which
can be formed by the conventional electron beam lithography is at
most about 1000 .ANG.. It is extremely difficult to form a pattern
of an ultrafine width of about 100 .ANG..
On the other hand, according to a method for forming a resist by an
electron beam which has already been proposed by the same inventors
as the present invention, although it is not publicly known
(hereinafter, the resist formed by this method is referred to as an
EBIR (Electron Beam Induced Resist)), by irradiating an electron
beam whose beam diameter was finely converged onto the substrate in
the atmosphere containing a raw material gas such as
alkylnaphthalene, a resist pattern of an ultrafine width comprising
amorphous hydrocarbon can be formed by the dry process. However, it
is difficult to remove such a resist pattern comprising amorphous
hydrocarbon unless the dry etching such as a reactive ion etching
(RIE) is used. However, since there is a fear of occurrence of
damages on the substrate and the like upon such a dry etching, a
method for forming the resist pattern which can be removed by the
wet process is desired.
As methods for forming a pattern, there has been known a method
whereby by irradiating an electron beam onto a film to be etched in
a carbonaceneous gas atmosphere, a carbonaceneous mask coating film
is formed on the film to be etched (Japanese Patent Laid Open
Publication No. Sho 61-0241). Also known is a method whereby a gas
containing a deposition material as a component element is supplied
onto a substrate which is cooled to 10.degree. C. or less, an
electron beam is irradiated to a desired portion of the surface of
the substrate, and the material is deposited onto the substrate
(Japanese Patent Laid Open Publication No. Sho 62-42417). However,
in these literatures publications there is no disclosure concerning
the optimum conditions for forming a resist pattern of an ultrafine
width.
OBJECTS AND SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide a
method for forming a pattern which can form a resist pattern of an
ultrafine width on the basis of the dry process.
It is another object of the present invention to provide a method
for forming a pattern which can form a resist pattern of an
ultrafine width in a relatively short time.
It is a further object of the present invention to provide a method
for forming a pattern which can form in stable fashion a resist
pattern of an ultrafine width.
It is a still further object of the present invention to provide a
method for forming a pattern wherein the pattern can be easily
removed by the wet process.
According to one aspect of the present invention, there is provided
a method for forming a pattern in which by selectively irradiating
a charged particle beam onto a substrate in an atmosphere
containing a raw material gas, a resist pattern comprising a
material which is produced on the substrate from the raw material
gas is formed. A pressure of the raw material gas is set to
10.sup.-7 to 10.sup.-5 Torr, an accelerating voltage of the charged
particle beam is set to 0.5 to 6 kV, and a beam current of the
charged particle beam is set to 10.sup.-13 to 10.sup.-7 A.
The upper limit of the pressure of the raw material gas is set
because when the pressure of the raw material gas is too high, the
raw material gas flows into the generating source side of the
charged particle beam and the pressure near the generating source
of the charged particle beam rises, so that there is a fear of
occurrence of a damage of the generating source of the charged
particle beam. On the other hand, the lower limit of the pressure
of the raw material gas is set in order to assure the resist growth
rate of a predetermined value or more; and because when the
pressure is set to a low value, there is no meaning in
consideration of the ultimate pressure in a specimen chamber before
the raw material gas is introduced. On the other hand, the upper
limit of the accelerating voltage of the charged particle beam is
set because when the accelerating voltage is 6 kV or more, the
multiple scattering and backscattering of the charged particles
upon irradiation of the charged particle beam becomes remarkable.
The lower limit of the accelerating voltage is set because when the
accelerating voltage is 0.5 kV or less, it is difficult to control
the charged particle beam. On the other hand, the upper limit of
the beam; current of the charged particle beam is set in
consideration of a performance of the generating source of the
charged particle beam and its lower limit is set in order to assure
the resist growth rate of a predetermined value or more.
As a charged particle beam; an electron beam, a positron beam, a
muon beam, or the like can be used. In the case of using the
electron beam, it is preferable to use a field emission gun which
can generate an electron beam having good coherence.
Since the beam diameter of the charged particle beam can be set to
an extremely small value, a pattern of an ultrafine width can be
formed. In this case, since the pressure of the raw material gas is
10.sup.-7 Torr or more and the beam current of the charged particle
beam is 10.sup.-13 A or more, a resist growth rate of a
predetermined value or more can be obtained. Therefore, the resist
pattern can be formed in a relatively short time. In addition,
since the pressure of the raw material gas is 10.sup.-5 Torr or
less and the accelerating voltage of the charged particle beam is
0.5 kV or more, the instability of the charged particle beam and
the occurrence of damage of the generating source due to the high
pressure near the generating source of the charged particle beam
are eliminated and the controllability of the charged particle beam
is also good. Therefore, since the charged particle beam can be
stably irradiated, the resist pattern can be formed in stable
fashion.
According to another aspect of the present invention there is
provided a method for forming a pattern in which by selectively
irradiating a charged particle beam onto a substrate in an
atmosphere containing a gaseous negative type resist, a
cross-linking reaction of negative type resist molecules adsorbed
on the surface of the substrate is caused, thereby forming a
pattern comprising the negative type resist molecules which caused
the cross-linking reaction.
As a negative type resist, it is possible to use a negative type
resist for the electron beam such as epoxidated polybutadiene
(EPB), poly (glycidyl methacrylate) (PGMA), poly (glycidyl
methacrylate-ethylacrylate copolymer) (P(GMA-EA)), material (CER)
in which methacrylic acid was reacted to ternary polymer of MMA,
GMA, and EA, chloro methylated polystyrene (CMS), poly (glycidyl
methacrylate-styrene copolymer) (P(GMA-St)), poly (glycidyl
methacrylate-3-chlorostyrene copolymer) (P(GMA-3Cl St)),
methylmaleic acid additive (SEL-N) of PGMA, polyethyl vinyl ether
(CEVE), vinyl ether copolymer (CVE), polysiloxane (PSi), poly
4-chlorostyrene (P4Cl St), polyvinyl benzyl chloride (PVBCl),
iodinated polystyrene (I-PS), chloromethylated
poly-.alpha.-methylstyrene (.alpha.M-CMS), polystyrene
tetrathiafulbalene (PS-TTF), etc. Among those negative type
resists, CMS, P(GMA-St), P(GMA-3Cl St), PSi, P4Cl St, PVBCl , I-PS,
.alpha.M-CMS, and PS-TTF are excellent in terms of the withstanding
property against the dry etching. Sensitivities (.mu.C/cm.sup.2) of
those negative type resists will now be shown in parentheses after
the resist names hereinafter EPB (0.05), PGMA (0.1), P(GMA-EA)
(0.34), CER (0.3), CMS (0.4-1.2), P(GMA-St) (2.6), P(GMA-3Cl St)
(2.8), SEL-N (0.3), CEVE (2), CVE (0.25), PSi (1), P4Cl St (2.5),
PVBCl (0.46), I-PS (1.5), .alpha.M-CMS (8.2), and PS-TTF (6).
In the atmosphere containing the gaseous negative type resist, the
molecules of the negative type resist are adsorbed on the surface
of the substrate. When the charged particle beam is irradiated onto
the substrate on which the negative type resist molecules are
adsorbed, the adsorbed negative type resist molecules cause the
cross-linking reaction. Thus, the pattern comprising the negative
type resist molecules which caused the cross-linking reaction is
formed on the substrate. A width of the pattern is determined by
the beam diameter of the charged particle beam and the size of blur
in the irradiation region due to the backscattering of the charged
particles from the substrate. The problem of the resolution
deteriorating due to the multiple scattering of the electrons in
the electron beam resist as in the conventional technique does not
occur. Therefore, a pattern of an ultrafine width of about 100
.ANG. can be easily formed. On the other hand, the pattern
comprising the negative type resist molecules which caused the
cross-linking reaction can be easily removed by the wet process
using a solvent of the negative type resist.
The above, and other, objects, features and advantages of the
present invention will become readily apparent from the following
detailed description thereof which is to be read in connection with
the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross sectional view showing a pattern forming
apparatus which is used in an embodiment of the present
invention;
FIGS. 2A to 2D show perspective views for explaining a method for
forming a pattern according to an embodiment of the present
invention in accordance with a step sequence; and
FIGS. 3A to 3D are perspective views showing another embodiment of
the present invention in accordance with a step sequence.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
An embodiment of the present invention will be described
hereinbelow with reference to the drawings. The embodiment shows an
embodiment in the case where the invention is applied to a method
for forming a pattern using direct writing by an electron beam.
FIG. 1 shows a pattern forming apparatus which is used in an
embodiment of the present invention.
As shown in FIG. 1, the pattern forming apparatus has a direct
writing apparatus 1 using an electron beam having a construction
similar to a scanning electron microscope (SEM). The direct writing
apparatus 1 has a barrel 2 and a specimen chamber 3. A field
emission gun 4 which can generate an electron beam having a good
coherence, a condenser lens 5, and a deflecting lens 6 are provided
in the barrel 2. The deflecting lens 6 is controlled by an electron
beam controller 7.
On the other hand, a susceptor 8 is provided in the specimen
chamber 3 and a substrate 9 is arranged on the susceptor 8. For
instance, the susceptor 8 is constructed such that it can be heated
or cooled by a temperature controller 10 comprising a resistance
heater and a Peltier cooler (cooler using the Peltier effect), so
that the temperature of the substrate 9 can be set to a
predetermined temperature. An electron beam 11 which is generated
from the field emission gun 4 is converged by the condenser lens 5.
Thereafter, the converged electron beam 11 is deflected by the
deflecting lens 6 controlled by the electron beam controller 7,
thereby scanning on the substrate 9 arranged on the susceptor
8.
Reference numeral 12 denotes a vessel in which a resist raw
material is enclosed. The resist raw material in the vessel 12 can
be introduced into the specimen chamber 3 through a conduit 13. For
instance, alkylnapthalene can be used as a resist raw material.
Reference numeral 14 denotes a mass flow controller to adjust a
flow rate of the resist raw material. Although alkylnapthalene
exists in a liquid form at ordinary temperature, it can be easily
gasified in the specimen chamber 3 held in the vacuum.
In the embodiment, the vacuum evacuation of the specimen chamber 3
is executed by a turbo molecular pump 16 coupled with the specimen
chamber 3 through a flexible tube 15 having a large diameter and an
sufficiently high conductance. A rotary pump 17 is connected to the
exhaust side of the turbo molecular pump 16. The turbo molecular
pump 16 is enclosed by a shield 18 to shield the magnetic field
which is generated from a motor to drive the turbo molecular pump
16. For instance, a magnetic material such as permalloy is used for
the material of the shield 18. In a manner similar to the above,
the vacuum evacuation of the barrel 2 is executed by a turbo
molecular pump 20 coupled with the upper portion of the barrel 2
through a flexible tube 19. Reference numeral 21 denotes a rotary
pump connected to the exhaust side of the turbo molecular pump 20.
Reference numeral 22 denotes a shield to shield the magnetic field
which is generated from a motor to drive the turbo molecular pump
20. The shield 22 is made of a magnetic material such as permalloy
similar to the shield 18.
In the embodiment, the vibration of the direct writing apparatus 1
in which the turbo molecular pump 16 is a vibrating source is the
largest cause of deterioration of the accuracy of writing by the
electron beam 11. Therefore, to prevent it, a spring constant k of
the flexible tube 15 is selected so as to satisfy the following
equation. ##EQU1## where d is a beam diameter of the electron beam
11, m is a mass of the direct writing apparatus 1, .omega. is an
angular frequency of the vibration of the turbo molecular pump 1,
and A is an amplitude of the vibration of the turbo molecular pump
16. By selecting the spring constant k of the flexible tube 15 so
as to satisfy the equation (1), the vibration of the direct writing
apparatus 16 which is caused by the vibration of the turbo
molecular pump 16 can be remarkably reduced. The reasons will now
be described hereinbelow.
The equation of the forced vibration of the direct writing
apparatus 1 in which the turbo molecular pump 16 is a vibrating
source is
where x denotes a displacement of the direct writing apparatus 1 in
the horizontal direction from the equilibrium position and t
denotes time. Assuming that x=Be.sup.i.omega.t (B is an amplitude
of the vibration of the direct writing apparatus 1), in the
equation (2),
By rearranging the equation (3),
is obtained. Thus, ##EQU2## The condition to execute the writing by
the electron beam 11 at the dimensional accuracy which is equal to
or less than the beam diameter d is B.ltoreq.d. In this case, from
the equation (5), ##EQU3## Now, when it is assumed that
(m/k).sup..omega.2 >1 and 1 is ignored for (m/k).sup..omega.2,
the condition of the equation (6) is as follows: ##EQU4## By
modifying the equation (7), the equation (1) is derived.
A practical example of the calculations of the spring constant k
will now be shown below. The amplitude of the vibration upon
operation of the turbo molecular pump 16, that is, A in the
equation (1) can be regarded to be 1 .mu.m=10.sup.-6 m or less. On
the other hand, assuming that the rotational speed of the turbo
molecular pump 16 is, for instance, 40000 r.p.m.,
.omega.=2.pi..times.(40000/60).about.4.times.10.sup.3 rad/sec. The
mass m of the direct writing apparatus 1 is set to, for instance,
about 100 kg. On the other hand, d=10 .ANG.=10.sup.-9 m by
considering the case where electron beam 11 was converged to the
smallest diameter. By substituting those numerical values for the
equation (1), ##EQU5##
A method for forming a resist pattern according to the embodiment
will now be described.
In FIG. 1, the specimen chamber 3 is previously evacuated to a high
vacuum (for instance, about 3.times.10.sup.-7 Torr) by the turbo
molecular pump 16. In this state, the resist raw material in the
vessel 12 is introduced into the specimen chamber 3 through the
conduit 13 while controlling the flow rate by the mass flow
controller 14. The pressure of the resist raw material gas in the
specimen chamber 3 is set to a value within a range from 10.sup.-7
to 10.sup.-5 Torr, for instance about 10.sup.-6 Torr. The substrate
9 such as a gallium arsenide (GaAs) substrate has previously been
arranged on the susceptor 8 in the specimen chamber 3 and is held
at a predetermined temperature by the temperature controller 10. As
shown in FIG. 2A, in this case it is assumed that a metal film 23
such as a tungsten (W) film has been formed on the substrate 9.
After the pressure of the resist raw material gas in the specimen
chamber 3 reached a predetermined value, the electron beam 11 is
generated by the field emission gun 4. The electron beam 11 is
scanned on the metal film 23 in the resist raw material gas
atmosphere under control by the electron beam controller 7, thereby
writing a predetermined pattern. In this case, the accelerating
voltage of the electron beam 11 is set to a value within a range
from 0.5 to 6 kV. The beam current is set to a value within a range
from 10.sup.-13 to 10.sup.-7 A. Furthermore, the beam diameter of
the electron beam 11 is set to, for instance, about 100 .ANG..
In the above resist raw material gas atmosphere, the resist raw
material gas molecules are adsorbed onto the surface of the metal
film 23. When the electron beam 11 is irradiated onto the resist
raw material molecules which are adsorbed, the resist molecules in
the portion to which the electron beam 11 was irradiated are
transformed to amorphous hydrocarbon. Thus, the material comprising
amorphous hydrocarbon is produced on the metal film 23 in the same
shape as the writing pattern of the electron beam 11. Due to this,
a resist pattern 24 comprising amorphous hydrocarbon is formed. The
resist pattern 24 comprising amorphous hydrocarbon has an excellent
withstanding property against the dry etching. Since a thickness of
the resist pattern 24 which is formed by the single writing
operation by the electron beam 11 is ordinarily small, the process
to produce hydrocarbon by irradiating the electron beam 11 into the
resist raw material molecules which are adsorbed onto the resist
pattern 24 which has once been formed is repeated. The resist
pattern 24 having a predetermined thickness is thus formed. FIG. 2B
shows such a state. In this case, on the metal film 23 of the
portion to which the electron beam 11 is not irradiated, when the
resist raw material molecules adsorbed are formed as a few atomic
layers, the adsorption of the resist raw material molecules is
saturated and the adsorption does not occur any more. Therefore,
the growth of the resist in the portion to which the electron beam
11 is not irradiated can be ignored.
After the resist pattern 24 of a predetermined thickness is formed
as shown in FIG. 2B, the metal film 23 is anisotropically etched in
the direction perpendicular to the substrate surface by, for
instance, the RIE using the resist pattern 24 as a mask. Thus, as
shown in FIG. 2C, a metal ultrafine line 25 having the same shape
as the resist pattern 24 and whose width is, for instance, about
100.ANG. is formed. Thereafter, the resist pattern 24 is etched off
and a state as shown in FIG. 2D is obtained.
The metal ultrafine line 25 can be used as a Schottky gate
electrode of a Schottky gate FET such as a GaAs MESFET or an HEMT
(High Electron Mobility Transistor), a wiring, or the like. If the
metal ultrafine line 25 is used as a Schottky gate electrode, an
FET whose transconductance gm is extremely high and which can
operate at a superhigh speed can be realized.
As mentioned above, according to the embodiment, the writing by the
electron beam 11 is executed under the conditions that the pressure
of the resist raw material gas is 10.sup.-7 to 10.sup.-5 Torr, the
accelerating voltage of the electron beam 11 is 0.5 to 6 kV, and
the beam current is 10.sup.-13 to 10.sup.-7 A. Therefore, the
occurrence of damage of the field emission gun 4 and the
instability of the electron beam 11 do not occur and the growth
rate of the resist is also relatively high. Consequently, the
resist pattern 24 of an ultrafine width can be formed in stable
fashion in a relatively short time.
On the other hand, since the spring constant k of the flexible tube
15 connecting the direct writing apparatus 1 and the turbo
molecular pump 16 is selected so as to satisfy the equation (1),
the vibration of the direct writing apparatus 1 which is caused by
the vibration due to the operation of the turbo molecular pump 16
greatly decreases. Therefore, the oscillation of the electron beam
11 is greatly reduced, so that the direct writing by the electron
beam 11 can be executed at high accuracy. On the other hand, since
the resist pattern 24 can be formed by the single step of direct
writing by the electron beam 11, the steps which are necessary to
form the resist pattern 24 can be remarkably reduced as compared
with that in the conventional apparatus.
Further, in the embodiment, since the writing is performed by using
the electron beam 11 of good coherence which is generated from the
field emission gun 4, the resist pattern 24 can be formed at a
resolution of about tens of .ANG.. Thus, the metal ultrafine line
25 can be formed as mentioned above. More generally speaking, for
instance, an ultrafine structure of smaller dimensions than the De
Broglie wavelength (about hundreds of .ANG.) of the electrons in
the semiconductor can be formed. Therefore, a device using the
quantum effect or the like can be realized.
On the other hand, since the vacuum evacuation of the specimen
chamber 3 is executed by the oil-free turbo molecular pump 16, the
resist pattern 24 can be formed in a clean vacuum where no oil
molecules exist.
Another embodiment of the present invention will now be
described.
FIGS. 3A to 3D show another embodiment of the present invention in
accordance with the step sequence.
In the embodiment, as shown in FIG. 3A, first there is prepared a
substrate 9 such as a GaAs substrate on which a metal film 23 such
as a W film is formed. Then, a gaseous negative type resist for an
electron beam is introduced into the specimen chamber 3 of the
pattern forming apparatus shown in FIG. 1. A vapor pressure of the
negative type resist is preferably set to, for instance, about
10.sup.-6 Torr. Although the negative type resist for the electron
beam exists as a liquid form in the atmosphere, it is easily
gasified in the vacuum. In the gaseous negative type resist
atmosphere, a negative type resist molecular layer 26 comprising
negative type resist molecules adsorbed on the surface of the metal
film 23 is formed. Next, in the specimen chamber 3, an electron
beam 11 whose beam diameter was finely converged to, for instance,
about 100 .ANG. is irradiated onto the metal film 23 in the gaseous
negative type resist atmosphere while scanning with a predetermined
pattern. An accelerating voltage of the electron beam 11 is set to,
for example, about 6 kV. On the other hand, a beam current of the
electron beam 11 is selected so as to obtain an irradiation charge
density which is substantially equivalent to, for instance, the
sensitivity of the negative type resist which is used.
The molecules in the negative type resist molecular layer 26 of the
portion to which the electron beam 11 was irradiated causes the
cross-linking reaction. Reference numeral 27 indicates a portion
comprising the negative type resist molecules which caused the
cross-linking reaction. Since a thickness of the negative type
resist molecular layer which caused the cross-linking reaction, and
which can be formed by one scan of the electron beam 11, is small,
the foregoing scan of the electron beam 11 is repetitively executed
until the thickness of the portion 27 comprising the negative type
resist molecules which caused the bridge reaction reaches a desired
thickness. Practically speaking, in a case of irradiating the
electron beam 11 at the charge density of, for instance, about 0.1
pC/cm, the scan of the electron beam 11 is repeated about 10.sup.4
to 10.sup.5 times.
FIG. 3B shows a state in which the portion 27, comprising the
negative type resist molecules which caused the bridge reaction has
been formed to a desired thickness by repetitively executing the
scan of the electron beam 11 as mentioned above.
Then the negative type resist molecular layer 26 to which the
electron beam 11 is not irradiated therefore the portions of the
layer 26 which does not cause the bridge reaction is selectively
removed by the wet process using the solvent for the negative type
resist. Thus, as shown in FIG. 3C, a resist pattern 24 comprising
the negative type resist molecules which caused the bridge reaction
is formed. A width of the resist pattern 24 is substantially equal
to the beam diameter of the electron beam 11 and, practically
speaking, it is set to, for instance, about 100 .ANG..
Next, the metal film 23 is anisotropically etched in the direction
perpendicular to the substrate surface by, for instance, the RIE by
using the resist pattern 24 as a mask. Thus, as shown in FIG. 3D, a
metal ultrafine line 25 of a width of, for instance, about 100
.ANG. is formed. Thereafter, the resist pattern 24 is removed by
the wet process using the solvent.
Under the irradiating conditions of the electron beam 11 mentioned
above, the EBIR comprising amorphous hydrocarbon which has already
been mentioned is not formed. This is because of the typical
conditions which are used to form the EBIR, that is that the beam
current of the electron beam 11 is set to 10.sup.-11 A, the
irradiating time is set to 10 seconds, the irradiation length is
set to 100 (.mu.m), the irradiation width is set to 1000 (.ANG.),
and the irradiation charge density is set to ##EQU6## This is
larger by about three to four orders of magnitude than the
sensitivity of the negative type resist for the electron beam which
has already been mentioned.
As mentioned above, according to the embodiment, since the electron
beam 11 is irradiated onto the metal film 23 in the gaseous
negative type resist atmosphere, the resist pattern 24 of the
ultrafine width which is substantially determined by the beam
diameter of the electron beam 11 can be formed. By etching the
metal film 23 by using the resist pattern 24 as a mask, the metal
ultrafine line 25 can be formed. On the other hand, since the
resist pattern 24 can be easily removed by the solvent in the wet
process, there is no fear of occurrence of damage on the substrate
9 and the like upon removal of the resist pattern 24. Further, the
so-called lift-off process can also be realized.
Although the embodiments of the invention have been described
above, the invention is not limited to the above embodiments, and
various types of modifications ar possible on the basis of the
technical idea of the invention.
For instance, the invention can be also obviously applied to form a
gate electrode of an MISFET, and can be also applied to the case
where, for example, a semiconductor substrate is etched so as to
obtain an ultrafine width, thereby realizing a quasi
one-dimensional FET. Further, the invention can be applied to the
case of forming an ultrafine pattern onto an arbitrary substrate
other than the semiconductor substrate.
Also, in the pattern forming apparatus shown in FIG. 1, a diffusion
pump can be also connected to the exhaust sides of the turbo
molecular pumps 16 and 20. On the other hand, the vacuum evacuation
of the barrel 2 can be also executed by, for example, an ion
pump.
According to the present invention, a resist pattern of an
ultrafine width can be stably formed in a relatively short time.
Further, according to the present invention, it is possible to form
a pattern of an ultrafine width which can be easily removed by the
wet process.
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